The present invention relates to apparatus and methods for electromagnetic interference shielding and, more particularly, to apparatus and methods for sealing apertures created by connectors in shielded enclosures.
There are many systems with very high frequency clocks and oscillators that generate high frequency emissions which radiate out from circuit cards and then out of the electronic shielded enclosures through the connector apertures, which are the largest apertures in shielded enclosures. The use of EMI shielded enclosures made of metallic materials or coated with metallic material is very commonly used in aerospace applications for the control of radiated emissions. Electromagnetic interference (EMI) shielding by a metallic wall is very effective, even for very thin walls, such as sprayed or brushed on metallic coats or foil sheets. The equation for shielding effectiveness is given by the following formula (I)SE=A+R−B  (I)whereSE is the shielding effectiveness of the metal shield,A=absorption loss,R=reflection loss, andB=multiple reflection loss.
The multiple reflection loss is only applicable to very thin metallic sheets, such as aluminum foil or spray on metallic coatings. The shielding effectiveness of a thin foil sheet is shown in FIG. 1. Note that the near field is considered when distance from the source to the shield is less than λ/2π. Even at the highest frequency of interest of approximately 1 gigahertz (GHz), λ/2π≈1.9 inches. So the shielded enclosure walls are in the near field of sources within the enclosure.
Sources can be either electric, such as high impedance voltage sources, or magnetic, such as low impedance current loops, but most sources are neither purely electric nor magnetic. Note that in FIG. 1, the near field magnetic attenuation is very low. However, most sources of interest are primarily electric, such as high impedance clock traces. For these primarily electric field sources, the aluminum shield provides a very high degree of attenuation, as compared to the far field plane wave attenuation. Thus, using the far field plane wave attenuation provides a good safety margin for most noise sources encountered. This would not be the case for low frequency magnetic fields.
One of the greatest limitations of metallic shielded enclosures is the input/output (I/O) interfaces. The connectors and other apertures required for I/O signals to enter and exit the shielded enclosure create breaches in the shielded enclosure, allowing the electromagnetic energy to enter and exit the shielded enclosure. Connectors typically have a dielectric insert where the connector pins are mounted. This insert creates an aperture with an electrical length equal to the greatest dimension of the connector opening L1 as shown in FIG. 2A for a circular connector. This is not a problem for low frequency signals since the diameter is very small compared to the wavelength of the signal and the shielding effectiveness is governed by formula (II)SE=20 log(λ/2L)  (II)whereSE is the aperture shielding effectiveness,L is the longest dimension of the aperture,λ is c/f, wherec is the speed of light, andf is the frequency of the noise source.
Thus, as shown in FIG. 3, at low frequencies, connector apertures provide a greater shielding effectiveness than the metallic material plane wave attenuation. As the frequency increases, however, the shielding effectiveness of the connector aperture eventually decreases below the material attenuation and limits the maximum attenuation of the enclosure. Above the frequency where λ=2×L, the aperture will not provide any attenuation.
With the advent of higher and higher frequency systems, I/O apertures have become a greater source of radiation. Periodic signals expand into Fourier series expansions at harmonics of the primary frequency of the time domain signal. Therefore, periodic signals, such as clocks and switching sources, will have high frequency harmonics that will radiate out of the connector apertures with little or no attenuation. This effect could be mitigated by placing a metallic chassis ground ring over the connector aperture, as shown in FIG. 2B. By having many smaller holes, with a diameter L2, rather than one large hole, with a diameter L1, the shielding effectiveness of the aperture is increased.
The equation for the effects of multiple holes is formula (III) below. The composite aperture shielding effectiveness as compared to that of the single connector aperture is also shown in FIG. 3 for nineteen 60-mil apertures. The net increase in shielding effectiveness is 11.2 dB for this configuration.SE=20×log(λ/2L)−20×log(N1/2)  (III)whereSE is the composite aperture shielding effectiveness,L is the longest dimension of the individual apertures, andN is the number of apertures.
The aperture electromagnetic radiation leakage effect forces designers to address the radiation from I/O apertures. The most common way to address the I/O interface electromagnetic radiation leakage is with an EMI doghouse. The EMI doghouse is a method of closing off the aperture leakage with a secondary compartment within the shielded enclosure which has a metallic interface. The EMI doghouse has traditionally required the creation of a mechanical barrier that must be formed or machined into the housing. The interface must then be connectorized or fitted with feed through filters to pass the interconnect signals from the shielded portion of the enclosure to the unshielded portion. This can add a great deal of cost and complexity to the enclosure.
As can be seen, there is a need for mitigating the electrical radiation through connector apertures in shielded enclosures.